Multivibrator type oscillator



Feb. 6, 1951 J, MOORE 2,540,539

MULTIVIBRATOR TYPE OSCILLATOR Filed Oct. 11, 1947 @501 wag Emma PatentedFeb. 6, I951 MULTIVIBRATOR TYPE OSCILLATOR James R..Moore, Washington,D. 0., assignor'to Radio Corporation of America, ,a corporation voiDelaware .Application october :11, 1947, Serial No. 779,283

invention :relates to oscillators of the multivibrator type, and moreparticularly to the stabilization of such oscillators.

The :principalobject of the invention is to proyide multivibrators inwhich the frequency and the amplitude-of the output voltage are .sub-.stantially independent of ordinary variations in characteristics-of thecomponent vacuum tubes.

Another. object :is to provide oscillators of the described type whereinthe frequency of oper- I ation can be controlled by :means of .a biasvolt- .age.

Itively, and grid leaks Hand I3. The plate of the tube 1 is coupled tothe grid of the tube 3 through a capacitor I5. The plate .of the tube .3may be likewise coupled .to the grid of the tube I through a capacitorI'.'I., .although this is not strictly essential to the operation of thecircuit. Both tubes I and 3 conduct .initially when the plate supplyvoltage is turned on. The potentials at both, plates decreases, owing todrop in the fi separate plate load resistors .I and 9, respecloadresistors I and .9, and .at the same time the cathodes becomeincreasingly positive with respect to ground because the. plate currentsof both tubes flow through the common cathode resistor 5. The decreasein potential at .each plate is communicated through the respectivecoupling condenser I5 or I! to the grid of the other tube. This,together with the positive cathode potential, tends to drive both tubestoward plate current cutoff.

I4Any slight difierence between the tubes 1 and {or their associatedcircuit elements causes one oI the tubes, for example the tube I, to.cut .off first. The plate current of the other tube '3 will thenincrease suddenly, causing a further sud .den decrease in the potentialat its plate. The

resulting negative pulse appearing at the grid of the tube I by way ofthe condenser I"! drives theltiibe 1 still lurtherbeyond cutofii.

tube 3 is now fully-conducting and the 3 Claims. (01. 25036) tube .I iswell below cutoff. This condition sists while the condenser I1discharges, through the resistor II to a potential approximating thatbetween the plate of the tube 3 and ground. This.

:allows the grid of the tube 1, whichwas-driven negative by the pulsefrom the plateof 'the tnhe to approach ground potential at arate muchdepends upon the capacitance of the condenser 41 and the resistance oftheeresistor I .I.

Finally the grid of the tube I vbecomes isufficiently less negative toallow conduction-to in the tube I. This increases the drop-fin iihcathode resistor 5 and. also applies-accegatire pulse to the grid of thetube 3.. These :gether operate to reduce the plate current imthe tube 3,causing an increase imthe potential-attire plate of the tube 3. Theincrea i y l flfiiliive voltage at the plate of the tube 3 drives thegrid of the tube I in a positive directiom'accelerating the increase-ofplate current inthe tube .1 and quickly driving the tube -3 beyondcutoff.

Conditions are now reversed, with .thetube I conducting and the tube 3cutofi. .The condenser i5 discharges through the resistor 1.3,,andthecycle is repeated. The tubes I .and 3 .conduct alternately, each for aperiod determined by the 3-6 product or time .constant .of itsrespective grid leak and condenser. The time required f or the actualtransfer of conduction .from one tube to the other is extremely small.Output may be taken from the plate of the tube 3asshown, or from theplate of the tube i, or from both tubes.

Oscillators of the above described type are used to produce sharplydefined non-sinusoidal waves for various applications. Often theiregu'ency is set approximately by the 3-0 networks and controlledexactly byinjectinga timing wave, for example at the control grid or oneof the tubes. The present invention contemplates oscillators of themultivibrator type which are 'suflicientl'y stable so that they willoperate at a definite selected frequency without being controlled by. aexternally produced timing wave.

The frequency of operation of an oscillator like that of :1, assuming notiming W611i) be applied, depends not only upon the time nonetants orthecoupling networks but also upon the voltage of the supply source and theindividual tube characteristics. The supply voltage determines howiarpositive the plate of either tube will go when the plate current isout off; the tube itself, as well as the supply voltage, determines howlow the potential at the plate will go when theitub si o ductinsMoreover. the potential at 'the plate or the tube vanes-11mins i Afurther tube 25 also has its cathode connected 3 the time that tube isconducting. All of these factors affect the length of the conductingperiods for each tube, and thus the frequency. The amplitude of theoutput wave and its shape are also functions of the supply voltage andof the tube characteristics.

According to the present invention, the negative grid swing, i. e. themost negative potential which can appear at the grid of either of themultivibrator tubes, is set at a definite level which does not depend onthe minimum plate potential at either tube and is preferably apredetermined fraction of the plate supply voltage.

Referring to Fig. 2, the tubes I and 3 are, asin Fig. 1, provided with acommon cathode resistor 5. The plate of the tube I is not coupleddirectly to the grid of the tube 3, however, but goes to a cathodefollower circuit IS. The cathode follower I9 includes a tube 2I with itsentire load resistor 23 in its cathode circuit. The. plate of the tube2| is connected directly to the plate supply source. The cathode of thetube. H is coupled through the condenser I5 to the'grid of the tube 3.

to t he resistor 23 and its anode connected to that of the tube 2|,- sothat the space discharge paths ofthe tubes 2| and 25 are'in parallel.The cohtrol grid of the'tube 25 .is biassed positive with respect toground by means of a voltage divider 3'0 21 connected across the platesupply source of the tubes I and 3. The'grid leaks and I3 ofthe-"tubes"I and 3 are not returned directly to ground'as in Fig. l, butare also biassed positive by a voltage divider 29, also connected acrossthe platesupply source.

A s'eco'ndoathode follower circuit 3I is included in the connectionbetween the plate of the tube 3 and the grid of the tube I. The circuit3I is substantially identical with the circuit I9, and includes'-corresponding tubes 2I' and 25' with a common cathode load resistor 23.The grid of the tubal? is biassed like that of the tube 25 by thevoltage divider 21.

1 The operation of the system of Fig. 2 is similar 'to that of Fig. 1,with certain significant exceptions The voltage divider 21 is adjustedto set the potential at the grids of the tubes 25 and 25' at a level E1of, say, 50 volts. The positive bias supplied by the voltage divider 29may be varied to adjust the frequency of oscillation to some extent,since this voltage will determine how far either ofthe couplingcondensers I5 and I! must discharge before the respective tube 3 or Iwill be able to start conducting.

Assume that the tube I has just started to con duct. The potential atits plate assumes a relativelylow value, less than the supply voltage bythe drop in the resistor I, and considerably less than'E1.' The actualpotential at the plate of the tube I thus depends not only on the platesupply voltage but also upon how much current the tube Lean conduct.

Aslong as the potential at the plate of the tube Iis greater than E1,the cathode follower tube 2! conducts just enoughto maintain the drop inits lead resistor 23 equal to that potentiaL How cver, as soon as thetube I conducts and its plate potential drops below E1, the tube 25starts conduotingiand provides a drop substantially equal to E1 acrossthe resistor 23. This, being considerably greater than the potential atthe grid of the tube2 I, cutsthelatter off, and any variationsintheplatepotential at the tube I have no effect '5 an the voltageacross the resistor 23.

Thus the voltage across the resistor 23 will have a substantially linearrectangular wave form, changing cyclically between 150 volts (the platesupply voltage) when the tube I is cut off and 50 volts (E1) when thetube I is conductive. These limits are determined solely by the supplyand bias voltages, and any normal variations in "the characteristics ofthe tubes I, 3, 2I and 25 will not change them. If the voltage divider21 is connected to the same source of supply as the plates of the tubesI and 3, the bias voltage E1 bear a constant ratio to the plate supplyvoltage regardless of variations in the latter. Thus I the maximumvoltage across the cathode follower 18 load will always be three (orsome other definite number) times the minimum voltage. This tends tomake the frequency of operation of the oscillator independent of normalsupply voltage variations.

It is apparent .without further discussion that the cathode followercircuit 3i acts exactly like the circuit I9, controlling accurately'thenegative swings of the voltage reaching the grid of the tube I.Besidesstabilizing the frequency of the oscillator, this keeps theamplitude substantially constant. The output may be taken as in 1 fromeither of the tubes I and 3, or may be taken off either of the cathodefollower load resistors. The latter is preferable because variations inthe load resistance at this point have substantially noefiect on thesystem.

Summarizing briefly, the invention has been described as an improvementin oscillators of the multivibrator type. The frequency of oscillationis stabilized by limiting the negative swings at the grids of both tubesto a definite value, preferably some predetermined fraction of the platesupply voltage.

"I claim as my invention: 1.- An oscillator including two electrondischarge tubes each including at least a cathode, a control grid, and aplate, a common cathode resistor connected to both of said cathodes andto ground; two cathode follower circuits, each in- 5 .'cluding an.electron discharge tube provided with a the cathode of each of saidcathode follower tubes to the control grid of the other of said firstmentioned tubes; two further tubes, each having its anode and itscathode connected to the anode and the cathode respectively of one ofsaid cath- ,ode follower tubes, and means biassing the control grids ofsaid further tubes to a potential in excess of the minimum voltage whichappears at f' the plates of said first mentioned tubes during operationof the system.

' .2. The invention as set forth in claim 1, in-

cluding meansv applying an adjustable positive bias to the control gridsof said first mentioned tubes to control the frequency of oscillationthereof.

{l ;3. A stabilized oscillator of the multivibrator type, including twoelectron discharge tubes with the plate circuit of each coupled to thegrid circuit of the other to produce and sustain oscillations comprisingalternate pulses of current in I. said tubes, and limiter meansconnected between the plate of each of said tubes and the grid of theother to prevent decrease below a predetermined value of the voltageapplied to either of said grids, said limited means comprising a cathodefollower circuit including a vacuum tube and 2,540,539 5 6 a cathodeoutput resistor and also including v REFERENCES CITED another Vacuumtube havmg an anode to QZ The following references are of record in theode space path that is connected in parallel confile of this patent.

ducting relation with the first mentioned vacuum tube of the cathodefollower circuit, said limiter 5 I, UNITED S A S PATENTS means furthercomprising means for biassing the Number Name Dat control grid of saidother tube to a p tential in 2,207,511 Geiger July 9, 1940 excessof theminimum voltage which appears 1 4 9 Dodington 25 1947 at the plates ofthe first mentioned l n 2,441,579 Kenyon May 18, 1948 charge tubesduring operation of the system. 10

JAMES, R. MOORE.

